Conventionally, ink containing a colorant, such as a dye, a pigment, or the like, a humectant, a penetrant, and water has been well known as ink used for inkjet recording. However, in the case where an image is formed with the ink containing the colorant on a recording medium, such as recording paper, or the like, the water-resistivity of the image is a matter of concern. Especially when an image is recorded with ink containing a dye on plain paper, the water-resistivity of the image is very poor. (The “plain paper” herein refers to paper which is one of various types of commercially-available paper, which is especially used for an electrophotographic copying machine, and which is produced without an intention to have an optimum structure, composition, properties, or the like, for inkjet recording.)
As disclosed in Japanese Unexamined Patent Publication No. 10-212439, Japanese Unexamined Patent Publication No. 11-293167, and Japanese Unexamined Patent Publication No. 11-315231, adding a hydrolyzable silane compound (organic silicon compound) to ink in order to improve the water-resistivity of an image formed on a recording medium has been conventionally proposed. When a drop of the ink is adhered onto a recording medium, and a solvent contained in the ink drop, such as water, or the like, evaporates or permeates into the recording medium, the silane compound is condensation-polymerized, and this condensation-polymerized silane compound encloses a colorant. Thus, even when the image formed on the recording medium is exposed to water, the colorant does not exude into the water. That is, the water-resistivity of the image is improved.
However, such ink containing a hydrolyzable silane compound is poor in long-term preservability. In general, the ink containing a hydrolyzable silane compound is generally neutral (at about pH 7). If the ink continues to absorb carbon dioxide in the air over a long time period, the ink becomes acidic. When the ink becomes acidic in this way, the silane compound gradually turns into a gel in water by condensation-polymerization, or the like. Especially when the ink is preserved in a high temperature environment of about 70° C., gelation of the silane compound is more accelerated. In the case where the ink containing the gelled silane compound is used for recording, insufficient ejection of ink drops occurs due to an increase of the viscosity of the ink, and accordingly, the quality of an image deteriorates. Further, in the case where a certain amount of silane compound has undergone a reaction, when an ink drop is adhered onto a recording medium and water in the ink drop evaporates or permeates into the recording medium, the silane compound does not sufficiently enclose a colorant through condensation-polymerization, and the water-resistivity of an image formed on a recording medium also deteriorates.
The present invention was conceived in view of the above problems. An objective of the present invention is to suppress deterioration of the quality of an image and maintain the water-resistivity of the image at a high level even when ink containing a water-soluble substance that is condensation-polymerized in the absence of water, such as a hydrolyzable silane compound, is used for recording after being preserved for a long time.